Slack, if I remember correctly, you do have to go back on the 'do not call' website periodically to keep it current. Isn't it something like once a year at least?

Granted I haven't used it in years, simply because if the calling # doesn't pop up with a name from my contact lists, I ain't answering it, and if they don't leave a voice mail, I ain't calling 'em back. Simple as that.
Of course my technique probably won't work for you because of your job....